Unconditionally use locked addition of zero to tip of the stack for memory barriers on i386. It works as a serialization instruction on all IA32 CPUs.
Unconditionally use locked addition of zero to tip of the stack for
memory barriers on i386. It works as a serialization instruction on
all IA32 CPUs.

Alternative solution of using {s,l,}fence requires run-time checking
of the presense of the corresponding SSE or SSE2 extensions, and
possible boot-time patching of the kernel text.

Suggested by:	many

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